Method and apparatus for adaptive tuning of wireless power transfer

ABSTRACT

Exemplary embodiments are directed to wireless power transfer. A transmit antenna generates an electromagnetic field at a resonant frequency of to create a coupling-mode region within a near field of the transmit antenna. A receive antenna receives the resonant frequency when it is within the coupling-mode region and resonates substantially near the resonant frequency. One, or both, of the transmit and receive antennas are tunable antennas that can be adaptively tuned. The adaptive tuning is accomplished by detecting a mismatch at the tunable antenna and generating a mismatch signal responsive to a voltage standing wave ratio at the tunable antenna. A resonance characteristic of the tunable antenna can be modified by adjusting a capacitance of a variable capacitor network connected to the tunable antenna.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

This application claims priority under 35 U.S.C. §119(e) to:

-   -   U.S. Provisional Patent Application 61/060,735 entitled “REVERSE        LINK SIGNALING VIA RECEIVE ANTENNA IMPEDANCE MODULATION” filed        on Jun. 11, 2008;    -   U.S. Provisional Patent Application 61/060,738 entitled        “SIGNALING CHARGING IN WIRELESS POWER ENVIRONMENT” filed on Jun.        11, 2008;    -   U.S. Provisional Patent Application 61/053,008 entitled        “ADAPTIVE TUNING MECHANISM FOR WIRELESS POWER TRANSFER” filed on        May 13, 2008;    -   U.S. Provisional Patent Application 61/053,010 entitled        “EFFICIENT POWER MANAGEMENT SCHEME FOR WIRELESS POWER CHARGING        SYSTEMS” filed on May 13, 2008;    -   U.S. Provisional Patent Application 61/060,741 entitled        “TRANSMIT POWER CONTROL FOR A WIRELESS CHARGING SYSTEM” filed on        Jun. 11, 2008;    -   U.S. Provisional Patent Application 61/053,000 entitled        “REPEATERS FOR ENHANCEMENT OF WIRELESS POWER TRANSFER” filed on        May 13, 2008;    -   U.S. Provisional Patent Application 61/053,004 entitled        “WIRELESS POWER TRANSFER FOR APPLIANCES AND EQUIPMENTS” filed on        May 13, 2008;    -   U.S. Provisional Patent Application 61/081,332 entitled        “WIRELESS POWER TRANSFER USING NEGATIVE RESISTANCE” filed on        Jul. 16, 2008;    -   U.S. Provisional Patent Application 61/053,012 entitled        “EMBEDDED RECEIVE ANTENNA FOR WIRELESS POWER TRANSFER” filed on        May 13, 2008; and    -   U.S. Provisional Patent Application 61/053,015 entitled “PLANAR        LARGE AREA WIRELESS CHARGING SYSTEM” filed on May 13, 2008.

REFERENCE TO CO-PENDING APPLICATIONS FOR PATENT

This application is also related to the following applications, whichare assigned to the assignee hereof and filed on even date herewith, thedisclosures of which are incorporated herein in their entirety byreference.

-   -   U.S. patent application Ser. No. 12/266,520 entitled “REPEATERS        FOR ENHANCEMENT OF WIRELESS POWER TRANSFER.”    -   U.S. patent application Ser. No. 12/267,041 entitled “WIRELESS        POWER TRANSFER FOR APPLIANCES AND EQUIPMENTS.”    -   U.S. patent application Ser. No. 12/266,525 entitled “METHOD AND        APPARATUS WITH NEGATIVE RESISTANCE IN WIRELESS POWER TRANSFERS.”

BACKGROUND

Typically, each battery powered device such as a wireless electronicdevice requires its own charger and power source, which is usually analternating current (AC) power outlet. Such a wired configurationbecomes unwieldy when many devices need charging.

Approaches are being developed that use over-the-air or wireless powertransmission between a transmitter and a receiver coupled to theelectronic device to be charged. Such approaches generally fall into twocategories. One is based on the coupling of plane wave radiation (alsocalled far-field radiation) between a transmit antenna and a receiveantenna on the device to be charged. The receive antenna collects theradiated power and rectifies it for charging the battery. Antennas aregenerally of resonant length in order to improve the couplingefficiency. This approach suffers from the fact that the power couplingfalls off quickly with distance between the antennas. So charging overreasonable distances (e.g., less than 1 to 2 meters) becomes difficult.Additionally, since the transmitting system radiates plane waves,unintentional radiation can interfere with other systems if not properlycontrolled through filtering.

Other approaches to wireless energy transmission techniques are based oninductive coupling between a transmit antenna embedded, for example, ina “charging” mat or surface and a receive antenna (plus a rectifyingcircuit) embedded in the host electronic device to be charged. Thisapproach has the disadvantage that the spacing between transmit andreceive antennas must be very close (e.g., within thousandths ofmeters). Though this approach does have the capability to simultaneouslycharge multiple devices in the same area, this area is typically verysmall and requires the user to accurately locate the devices to aspecific area. Therefore, there is a need to provide a wireless chargingarrangement that accommodates flexible placement and orientation oftransmit and receive antennas.

With wireless power transmission there is a need for systems and methodsfor adjusting the operating characteristics of the antennas to adapt todifferent circumstances and optimize power transfer characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified block diagram of a wireless power transfersystem.

FIG. 2 shows a simplified schematic diagram of a wireless power transfersystem.

FIG. 3 shows a schematic diagram of a loop antenna for use in exemplaryembodiments of the present invention.

FIG. 4 shows simulation results indicating coupling strength betweentransmit and receive antennas.

FIGS. 5A and 5B show layouts of loop antennas for transmit and receiveantennas according to exemplary embodiments of the present invention.

FIG. 6 shows simulation results indicating coupling strength betweentransmit and receive antennas relative to various circumference sizesfor the square and circular transmit antennas illustrated in FIGS. 5Aand 5B.

FIG. 7 shows simulation results indicating coupling strength betweentransmit and receive antennas relative to various surface areas for thesquare and circular transmit antennas illustrated in FIGS. 5A and 5B.

FIG. 8 shows various placement points for a receive antenna relative toa transmit antenna to illustrate coupling strengths in coplanar andcoaxial placements.

FIG. 9 shows simulation results indicating coupling strength for coaxialplacement at various distances between the transmit and receiveantennas.

FIG. 10 is a simplified block diagram of a transmitter, in accordancewith an exemplary embodiment of the present invention.

FIG. 11 is a simplified block diagram of a receiver, in accordance withan exemplary embodiment of the present invention.

FIG. 12 shows a simplified schematic of a portion of transmit circuitryfor carrying out messaging between a transmitter and a receiver.

FIGS. 13A-13C shows a simplified schematic of a portion of receivecircuitry in various states to illustrate messaging between a receiverand a transmitter.

FIGS. 14A-14C shows a simplified schematic of a portion of alternativereceive circuitry in various states to illustrate messaging between areceiver and a transmitter.

FIGS. 15A-15C are timing diagrams illustrating a messaging protocol forcommunication between a transmitter and a receiver.

FIGS. 16A-16D are simplified block diagrams illustrating a beacon powermode for transmitting power between a transmitter and a receiver.

FIG. 17A illustrates a large transmit antenna with a smaller repeaterantenna disposed coplanar with, and coaxial with, the transmit antenna.

FIG. 17B illustrates a transmit antenna with a larger repeater antennawith a coaxial placement relative to the transmit antenna.

FIG. 18A illustrates a large transmit antenna with a three differentsmaller repeater antennas disposed coplanar with, and within a perimeterof, the transmit antenna.

FIG. 18B illustrates a large transmit antenna with smaller repeaterantennas with offset coaxial placements and offset coplanar placementsrelative to the transmit antenna.

FIG. 19 shows simulation results indicating coupling strength between atransmit antenna, a repeater antenna and a receive antenna.

FIG. 20A shows simulation results indicating coupling strength between atransmit antenna and receive antenna with no repeater antennas.

FIG. 20B shows simulation results indicating coupling strength between atransmit antenna and receive antenna with a repeater antenna.

FIG. 21A-21C are simplified block diagrams of adaptive tuning circuitsfor an antenna using a T-network, an L-network, and a Pi-network,respectively.

FIG. 22 is a simplified block diagram of an adaptive tuning circuit fora transmit antenna based on power consumption at the transmit antenna.

FIGS. 23A and 23B are simplified circuit diagrams illustrating exemplaryembodiments of variable capacitor networks.

FIGS. 24A and 24B show simulation results for near field coupledtransmit and receive antennas before and after adaptive tuning,respectively.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of thepresent invention and is not intended to represent the only embodimentsin which the present invention can be practiced. The term “exemplary”used throughout this description means “serving as an example, instance,or illustration,” and should not necessarily be construed as preferredor advantageous over other exemplary embodiments. The detaileddescription includes specific details for the purpose of providing athorough understanding of the exemplary embodiments of the invention. Itwill be apparent to those skilled in the art that the exemplaryembodiments of the invention may be practiced without these specificdetails. In some instances, well-known structures and devices are shownin block diagram form in order to avoid obscuring the novelty of theexemplary embodiments presented herein.

The words “wireless power” is used herein to mean any form of energyassociated with electric fields, magnetic fields, electromagneticfields, or otherwise that is transmitted between from a transmitter to areceiver without the use of physical electromagnetic conductors.

FIG. 1 illustrates wireless transmission or charging system 100, inaccordance with various exemplary embodiments of the present invention.Input power 102 is provided to a transmitter 104 for generating aradiated field 106 for providing energy transfer. A receiver 108 couplesto the radiated field 106 and generates an output power 110 for storingor consumption by a device (not shown) coupled to the output power 110.Both the transmitter 104 and the receiver 108 are separated by adistance 112. In one exemplary embodiment, transmitter 104 and receiver108 are configured according to a mutual resonant relationship and whenthe resonant frequency of receiver 108 and the resonant frequency oftransmitter 104 are exactly identical, transmission losses between thetransmitter 104 and the receiver 108 are minimal when the receiver 108is located in the “near-field” of the radiated field 106.

Transmitter 104 further includes a transmit antenna 114 for providing ameans for energy transmission and receiver 108 further includes areceive antenna 118 for providing a means for energy reception. Thetransmit and receive antennas are sized according to applications anddevices to be associated therewith. As stated, an efficient energytransfer occurs by coupling a large portion of the energy in thenear-field of the transmitting antenna to a receiving antenna ratherthan propagating most of the energy in an electromagnetic wave to thefar field. When in this near-field a coupling mode may be developedbetween the transmit antenna 114 and the receive antenna 118. The areaaround the antennas 114 and 118 where this near-field coupling may occuris referred to herein as a coupling-mode region.

FIG. 2 shows a simplified schematic diagram of a wireless power transfersystem. The transmitter 104 includes an oscillator 122, a poweramplifier 124 and a filter and matching circuit 126. The oscillator isconfigured to generate at a desired frequency, which may be adjusted inresponse to adjustment signal 123. The oscillator signal may beamplified by the power amplifier 124 with an amplification amountresponsive to control signal 125. The filter and matching circuit 126may be included to filter out harmonics or other unwanted frequenciesand match the impedance of the transmitter 104 to the transmit antenna114.

The receiver may include a matching circuit 132 and a rectifier andswitching circuit to generate a DC power output to charge a battery 136as shown in FIG. 2 or power a device coupled to the receiver (notshown). The matching circuit 132 may be included to match the impedanceof the receiver 108 to the receive antenna 118.

As illustrated in FIG. 3, antennas used in exemplary embodiments may beconfigured as a “loop” antenna 150, which may also be referred to hereinas a “magnetic” antenna. Loop antennas may be configured to include anair core or a physical core such as a ferrite core. Air core loopantennas may be more tolerable to extraneous physical devices placed inthe vicinity of the core. Furthermore, an air core loop antenna allowsthe placement of other components within the core area. In addition, anair core loop may more readily enable placement of the receive antenna118 (FIG. 2) within a plane of the transmit antenna 114 (FIG. 2) wherethe coupled-mode region of the transmit antenna 114 (FIG. 2) may be morepowerful.

As stated, efficient transfer of energy between the transmitter 104 andreceiver 108 occurs during matched or nearly matched resonance betweenthe transmitter 104 and the receiver 108. However, even when resonancebetween the transmitter 104 and receiver 108 are not matched, energy maybe transferred at a lower efficiency. Transfer of energy occurs bycoupling energy from the near-field of the transmitting antenna to thereceiving antenna residing in the neighborhood where this near-field isestablished rather than propagating the energy from the transmittingantenna into free space.

The resonant frequency of the loop or magnetic antennas is based on theinductance and capacitance. Inductance in a loop antenna is generallysimply the inductance created by the loop, whereas, capacitance isgenerally added to the loop antenna's inductance to create a resonantstructure at a desired resonant frequency. As a non-limiting example,capacitor 152 and capacitor 154 may be added to the antenna to create aresonant circuit that generates resonant signal 156. Accordingly, forlarger diameter loop antennas, the size of capacitance needed to induceresonance decreases as the diameter or inductance of the loop increases.Furthermore, as the diameter of the loop or magnetic antenna increases,the efficient energy transfer area of the near-field increases. Ofcourse, other resonant circuits are possible. As another non-limitingexample, a capacitor may be placed in parallel between the two terminalsof the loop antenna. In addition, those of ordinary skill in the artwill recognize that for transmit antennas the resonant signal 156 may bean input to the loop antenna 150.

Exemplary embodiments of the invention include coupling power betweentwo antennas that are in the near-fields of each other. As stated, thenear-field is an area around the antenna in which electromagnetic fieldsexist but may not propagate or radiate away from the antenna. They aretypically confined to a volume that is near the physical volume of theantenna. In the exemplary embodiments of the invention, magnetic typeantennas such as single and multi-turn loop antennas are used for bothtransmit (Tx) and receive (Rx) antenna systems since magnetic near-fieldamplitudes tend to be higher for magnetic type antennas in comparison tothe electric near-fields of an electric-type antenna (e.g., a smalldipole). This allows for potentially higher coupling between the pair.Furthermore, “electric” antennas (e.g., dipoles and monopoles) or acombination of magnetic and electric antennas is also contemplated.

The Tx antenna can be operated at a frequency that is low enough andwith an antenna size that is large enough to achieve good coupling(e.g., >−4 dB) to a small Rx antenna at significantly larger distancesthan allowed by far field and inductive approaches mentioned earlier. Ifthe Tx antenna is sized correctly, high coupling levels (e.g., −2 to −4dB) can be achieved when the Rx antenna on a host device is placedwithin a coupling-mode region (i.e., in the near-field) of the driven Txloop antenna.

FIG. 4 shows simulation results indicating coupling strength betweentransmit and receive antennas. Curves 170 and 172 indicate a measure ofacceptance of power by the transmit and receive antennas, respectively.In other words, with a large negative number there is a very closeimpedance match and most of the power is accepted and, as a result,radiated by the transmit antenna. Conversely, a small negative numberindicates that much of the power is reflected back from the antennabecause there is not a close impedance match at the given frequency. InFIG. 4, the transmit antenna and the receive antenna are tuned to have aresonant frequency of about 13.56 MHz.

Curve 170 illustrates the amount of power transmitted from the transmitantenna at various frequencies. Thus, at points 1 a and 3 a,corresponding to about 13.528 MHz and 13.593 MHz, much of the power isreflected and not transmitted out of the transmit antenna. However, atpoint 2 a, corresponding to about 13.56 MHz, it can be seen that a largeamount of the power is accepted and transmitted out of the antenna.

Similarly, curve 172 illustrates the amount of power received by thereceive antenna at various frequencies. Thus, at points 1 b and 3 b,corresponding to about 13.528 MHz and 13.593 MHz, much of the power isreflected and not conveyed through the receive antenna and into thereceiver. However, at point 2 b corresponding to about 13.56 MHz, it canbe seen that a large amount of the power is accepted by the receiveantenna and conveyed into the receiver.

Curve 174 indicates the amount of power received at the receiver afterbeing sent from the transmitter through the transmit antenna, receivedthrough the receive antenna and conveyed to the receiver. Thus, atpoints 1 c and 3 c, corresponding to about 13.528 MHz and 13.593 MHz,much of the power sent out of the transmitter is not available at thereceiver because (1) the transmit antenna rejects much of the power sentto it from the transmitter and (2) the coupling between the transmitantenna and the receive antenna is less efficient as the frequenciesmove away from the resonant frequency. However, at point 2 ccorresponding to about 13.56 MHz, it can be seen that a large amount ofthe power sent from the transmitter is available at the receiver,indicating a high degree of coupling between the transmit antenna andthe receive antenna.

FIGS. 5A and 5B show layouts of loop antennas for transmit and receiveantennas according to exemplary embodiments of the present invention.Loop antennas may be configured in a number of different ways, withsingle loops or multiple loops at wide variety of sizes. In addition,the loops may be a number of different shapes, such as, for exampleonly, circular, elliptical, square, and rectangular. FIG. 5A illustratesa large square loop transmit antenna 114S and a small square loopreceive antenna 118 placed in the same plane as the transmit antenna114S and near the center of the transmit antenna 114S. FIG. 5Billustrates a large circular loop transmit antenna 114C and a smallsquare loop receive antenna 118′ placed in the same plane as thetransmit antenna 114C and near the center of the transmit antenna 114C.The square loop transmit antenna 114S has side lengths of “a” while thecircular loop transmit antenna 114C has a diameter of “Φ.” For a squareloop, it can be shown that there is an equivalent circular loop whosediameter may be defined as: Φ_(eq)=4a/π.

FIG. 6 shows simulation results indicating coupling strength betweentransmit and receive antennas relative to various circumferences for thesquare and circular transmit antennas illustrated in FIGS. 4A and 4B.Thus, curve 180 shows coupling strength between the circular looptransmit antennas 114C and the receive antenna 118 at variouscircumference sizes for the circular loop transmit antenna 114C.Similarly, curve 182 shows coupling strength between the square looptransmit antennas 114S and the receive antenna 118′ at variousequivalent circumference sizes for the transmit loop transmit antenna114S.

FIG. 7 shows simulation results indicating coupling strength betweentransmit and receive antennas relative to various surface areas for thesquare and circular transmit antennas illustrated in FIGS. 5A and 5B.Thus, curve 190 shows coupling strength between the circular looptransmit antennas 114C and the receive antenna 118 at various surfaceareas for the circular loop transmit antenna 114C. Similarly, curve 192shows coupling strength between the square loop transmit antennas 114Sand the receive antenna 118′ at various surface areas for the transmitloop transmit antenna 114S.

FIG. 8 shows various placement points for a receive antenna relative toa transmit antenna to illustrate coupling strengths in coplanar andcoaxial placements. “Coplanar,” as used herein, means that the transmitantenna and receive antenna have planes that are substantially aligned(i.e., have surface normals pointing in substantially the samedirection) and with no distance (or a small distance) between the planesof the transmit antenna and the receive antenna. “Coaxial,” as usedherein, means that the transmit antenna and receive antenna have planesthat are substantially aligned (i.e., have surface normals pointing insubstantially the same direction) and the distance between the twoplanes is not trivial and furthermore, the surface normal of thetransmit antenna and the receive antenna lie substantially along thesame vector, or the two normals are in echelon.

As examples, points p1, p2, p3, and p7 are all coplanar placement pointsfor a receive antenna relative to a transmit antenna. As anotherexample, point p5 and p6 are coaxial placement points for a receiveantenna relative to a transmit antenna. The table below shows couplingstrength (S21) and coupling efficiency (expressed as a percentage ofpower transmitted from the transmit antenna that reached the receiveantenna) at the various placement points (p1-p7) illustrated in FIG. 8.

TABLE 1 Efficiency (TX DC power in to Distance from S21 efficiency RX DCpower Position plane (cm) (%) out) p1 0 46.8 28 p2 0 55.0 36 p3 0 57.535 p4 2.5 49.0 30 p5 17.5 24.5 15 p6 17.5 0.3 0.2 p7 0 5.9 3.4

As can be seen, the coplanar placement points p1, p2, and p3, all showrelatively high coupling efficiencies. Placement point p7 is also acoplanar placement point, but is outside of the transmit loop antenna.While placement point p7 does not have a high coupling efficiency, it isclear that there is some coupling and the coupling-mode region extendsbeyond the perimeter of the transmit loop antenna.

Placement point p5 is coaxial with the transmit antenna and showssubstantial coupling efficiency. The coupling efficiency for placementpoint p5 is not as high as the coupling efficiencies for the coplanarplacement points. However, the coupling efficiency for placement pointp5 is high enough that substantial power can be conveyed between thetransmit antenna and a receive antenna in a coaxial placement.

Placement point p4 is within the circumference of the transmit antennabut at a slight distance above the plane of the transmit antenna in aposition that may be referred to as an offset coaxial placement (i.e.,with surface normals in substantially the same direction but atdifferent locations) or offset coplanar (i.e., with surface normals insubstantially the same direction but with planes that are offsetrelative to each other). From the table it can be seen that with anoffset distance of 2.5 cm, placement point p4 still has relatively goodcoupling efficiency.

Placement point p6 illustrates a placement point outside thecircumference of the transmit antenna and at a substantial distanceabove the plane of the transmit antenna. As can be seen from the table,placement point p7 shows little coupling efficiency between the transmitand receive antennas.

FIG. 9 shows simulation results indicating coupling strength for coaxialplacement at various distances between the transmit and receiveantennas. The simulations for FIG. 9 are for square transmit and receiveantennas in a coaxial placement, both with sides of about 1.2 meters andat a transmit frequency of 10 MHz. It can be seen that the couplingstrength remains quite high and uniform at distances of less than about0.5 meters.

FIG. 10 is a simplified block diagram of a transmitter, in accordancewith an exemplary embodiment of the present invention. A transmitter 200includes transmit circuitry 202 and a transmit antenna 204. Generally,transmit circuitry 202 provides RF power to the transmit antenna 204 byproviding an oscillating signal resulting in generation of near-fieldenergy about the transmit antenna 204. By way of example, transmitter200 may operate at the 13.56 MHz ISM band.

Exemplary transmit circuitry 202 includes a fixed impedance matchingcircuit 206 for matching the impedance of the transmit circuitry 202(e.g., 50 ohms) to the transmit antenna 204 and a low pass filter (LPF)208 configured to reduce harmonic emissions to levels to preventself-jamming of devices coupled to receivers 108 (FIG. 1). Otherembodiments may include different filter topologies, including but notlimited to, notch filters that attenuate specific frequencies whilepassing others and may include an adaptive impedance match, that can bevaried based on measurable transmit metrics, such as output power to theantenna or DC current draw by the power amplifier. Transmit circuitry202 further includes a power amplifier 210 configured to drive an RFsignal as determined by an oscillator 212. The transmit circuitry may becomprised of discrete devices or circuits, or alternately, may becomprised of an integrated assembly. An exemplary RF power output fromtransmit antenna 204 may be on the order of 2.5 Watts.

Transmit circuitry 202 further includes a processor 214 for enabling theoscillator 212 during transmit phases (or duty cycles) for specificreceivers, for adjusting the frequency of the oscillator, and foradjusting the output power level for implementing a communicationprotocol for interacting with neighboring devices through their attachedreceivers.

The transmit circuitry 202 may further include a load sensing circuit216 for detecting the presence or absence of active receivers in thevicinity of the near-field generated by transmit antenna 204. By way ofexample, a load sensing circuit 216 monitors the current flowing to thepower amplifier 210, which is affected by the presence or absence ofactive receivers in the vicinity of the near-field generated by transmitantenna 204. Detection of changes to the loading on the power amplifier210 are monitored by processor 214 for use in determining whether toenable the oscillator 212 for transmitting energy to communicate with anactive receiver.

Transmit antenna 204 may be implemented as an antenna strip with thethickness, width and metal type selected to keep resistive losses low.In a conventional implementation, the transmit antenna 204 can generallybe configured for association with a larger structure such as a table,mat, lamp or other less portable configuration. Accordingly, thetransmit antenna 204 generally will not need “turns” in order to be of apractical dimension. An exemplary implementation of a transmit antenna204 may be “electrically small” (i.e., fraction of the wavelength) andtuned to resonate at lower usable frequencies by using capacitors todefine the resonant frequency. In an exemplary application where thetransmit antenna 204 may be larger in diameter, or length of side if asquare loop, (e.g., 0.50 meters) relative to the receive antenna, thetransmit antenna 204 will not necessarily need a large number of turnsto obtain a reasonable capacitance.

FIG. 11 is a block diagram of a receiver, in accordance with anembodiment of the present invention. A receiver 300 includes receivecircuitry 302 and a receive antenna 304. Receiver 300 further couples todevice 350 for providing received power thereto. It should be noted thatreceiver 300 is illustrated as being external to device 350 but may beintegrated into device 350. Generally, energy is propagated wirelesslyto receive antenna 304 and then coupled through receive circuitry 302 todevice 350.

Receive antenna 304 is tuned to resonate at the same frequency, or nearthe same frequency, as transmit antenna 204 (FIG. 10). Receive antenna304 may be similarly dimensioned with transmit antenna 204 or may bedifferently sized based upon the dimensions of an associated device 350.By way of example, device 350 may be a portable electronic device havingdiametric or length dimension smaller that the diameter of length oftransmit antenna 204. In such an example, receive antenna 304 may beimplemented as a multi-turn antenna in order to reduce the capacitancevalue of a tuning capacitor (not shown) and increase the receiveantenna's impedance. By way of example, receive antenna 304 may beplaced around the substantial circumference of device 350 in order tomaximize the antenna diameter and reduce the number of loop turns (i.e.,windings) of the receive antenna and the inter-winding capacitance.

Receive circuitry 302 provides an impedance match to the receive antenna304. Receive circuitry 302 includes power conversion circuitry 306 forconverting a received RF energy source into charging power for use bydevice 350. Power conversion circuitry 306 includes an RF-to-DCconverter 308 and may also in include a DC-to-DC converter 310. RF-to-DCconverter 308 rectifies the RF energy signal received at receive antenna304 into a non-alternating power while DC-to-DC converter 310 convertsthe rectified RF energy signal into an energy potential (e.g., voltage)that is compatible with device 350. Various RF-to-DC converters arecontemplated including partial and full rectifiers, regulators, bridges,doublers, as well as linear and switching converters.

Receive circuitry 302 may further include switching circuitry 312 forconnecting receive antenna 304 to the power conversion circuitry 306 oralternatively for disconnecting the power conversion circuitry 306.Disconnecting receive antenna 304 from power conversion circuitry 306not only suspends charging of device 350, but also changes the “load” as“seen” by the transmitter 200 (FIG. 2) as is explained more fully below.As disclosed above, transmitter 200 includes load sensing circuit 216which detects fluctuations in the bias current provided to transmitterpower amplifier 210. Accordingly, transmitter 200 has a mechanism fordetermining when receivers are present in the transmitter's near-field.

When multiple receivers 300 are present in a transmitter's near-field,it may be desirable to time-multiplex the loading and unloading of oneor more receivers to enable other receivers to more efficiently coupleto the transmitter. A receiver may also be cloaked in order to eliminatecoupling to other nearby receivers or to reduce loading on nearbytransmitters. This “unloading” of a receiver is also known herein as a“cloaking.” Furthermore, this switching between unloading and loadingcontrolled by receiver 300 and detected by transmitter 200 provides acommunication mechanism from receiver 300 to transmitter 200 as isexplained more fully below. Additionally, a protocol can be associatedwith the switching which enables the sending of a message from receiver300 to transmitter 200. By way of example, a switching speed may be onthe order of 100 μsec.

In an exemplary embodiment, communication between the transmitter andthe receiver refers to a Device Sensing and Charging Control Mechanism,rather than conventional two-way communication. In other words, thetransmitter uses on/off keying of the transmitted signal to adjustwhether energy is available in the near-filed. The receivers interpretthese changes in energy as a message from the transmitter. From thereceiver side, the receiver uses tuning and de-tuning of the receiveantenna to adjust how much power is being accepted from the near-field.The transmitter can detect this difference in power used from the nearfield and interpret these changes as a message from the receiver.

Receive circuitry 302 may further include signaling detector and beaconcircuitry 314 used to identify received energy fluctuations, which maycorrespond to informational signaling from the transmitter to thereceiver. Furthermore, signaling and beacon circuitry 314 may also beused to detect the transmission of a reduced RF signal energy (i.e., abeacon signal) and to rectify the reduced RF signal energy into anominal power for awakening either un-powered or power-depleted circuitswithin receive circuitry 302 in order to configure receive circuitry 302for wireless charging.

Receive circuitry 302 further includes processor 316 for coordinatingthe processes of receiver 300 described herein including the control ofswitching circuitry 312 described herein. Cloaking of receiver 300 mayalso occur upon the occurrence of other events including detection of anexternal wired charging source (e.g., wall/USB power) providing chargingpower to device 350. Processor 316, in addition to controlling thecloaking of the receiver, may also monitor beacon circuitry 314 todetermine a beacon state and extract messages sent from the transmitter.Processor 316 may also adjust DC-to-DC converter 310 for improvedperformance.

FIG. 12 shows a simplified schematic of a portion of transmit circuitryfor carrying out messaging between a transmitter and a receiver. In someexemplary embodiments of the present invention, a means forcommunication may be enabled between the transmitter and the receiver.In FIG. 12 a power amplifier 210 drives the transmit antenna 204 togenerate the radiated field. The power amplifier is driven by a carriersignal 220 that is oscillating at a desired frequency for the transmitantenna 204. A transmit modulation signal 224 is used to control theoutput of the power amplifier 210.

The transmit circuitry can send signals to receivers by using an ON/OFFkeying process on the power amplifier 210. In other words, when thetransmit modulation signal 224 is asserted, the power amplifier 210 willdrive the frequency of the carrier signal 220 out on the transmitantenna 204. When the transmit modulation signal 224 is negated, thepower amplifier will not drive out any frequency on the transmit antenna204.

The transmit circuitry of FIG. 12 also includes a load sensing circuit216 that supplies power to the power amplifier 210 and generates areceive signal 235 output. In the load sensing circuit 216 a voltagedrop across resistor R_(s) develops between the power in signal 226 andthe power supply 228 to the power amplifier 210. Any change in the powerconsumed by the power amplifier 210 will cause a change in the voltagedrop that will be amplified by differential amplifier 230. When thetransmit antenna is in coupled mode with a receive antenna in a receiver(not shown in FIG. 12) the amount of current drawn by the poweramplifier 210 will change. In other words, if no coupled mode resonanceexist for the transmit antenna 210, the power required to drive theradiated field will be first amount. If a coupled mode resonance exists,the amount of power consumed by the power amplifier 210 will go upbecause much of the power is being coupled into the receive antenna.Thus, the receive signal 235 can indicate the presence of a receiveantenna coupled to the transmit antenna 235 and can also detect signalssent from the receive antenna, as explained below. Additionally, achange in receiver current draw will be observable in the transmitter'spower amplifier current draw, and this change can be used to detectsignals from the receive antennas, as explained below.

FIGS. 13A-13C shows a simplified schematic of a portion of receivecircuitry in various states to illustrate messaging between a receiverand a transmitter. All of FIGS. 13A-13C show the same circuit elementswith the difference being state of the various switches. A receiveantenna 304 includes a characteristic inductance L1, which drives node350. Node 350 is selectively coupled to ground through switch S1A. Node350 is also selectively coupled to diode D1 and rectifier 318 throughswitch S1B. The rectifier 318 supplies a DC power signal 322 to areceive device (not shown) to power the receive device, charge abattery, or a combination thereof. The diode D1 is coupled to a transmitsignal 320 which is filtered to remove harmonics and unwantedfrequencies with capacitor C3 and resistor R1. Thus the combination ofD1, C3, and R1 can generate a signal on the transmit signal 320 thatmimics the transmit modulation generated by the transmit modulationsignal 224 discussed above with reference to the transmitter in FIG. 12.

Exemplary embodiments of the invention includes modulation of thereceive device's current draw and modulation of the receive antenna'simpedance to accomplish reverse link signaling. With reference to bothFIG. 13A and FIG. 12, as the power draw of the receive device changes,the load sensing circuit 216 detects the resulting power changes on thetransmit antenna and from these changes can generate the receive signal235.

In the embodiments of FIGS. 13A-13C, the current draw through thetransmitter can be changed by modifying the state of switches S1A andS2A. In FIG. 13A, switch S1A and switch S2A are both open creating a “DCopen state” and essentially removing the load from the transmit antenna204. This reduces the current seen by the transmitter.

In FIG. 13B, switch S1A is closed and switch S2A is open creating a “DCshort state” for the receive antenna 304. Thus the state in FIG. 13B canbe used to increase the current seen in the transmitter.

In FIG. 13C, switch S1A is open and switch S2A is closed creating anormal receive mode (also referred to herein as a “DC operating state”)wherein power can be supplied by the DC out signal 322 and a transmitsignal 320 can be detected. In the state shown in FIG. 13C the receiverreceives a normal amount of power, thus consuming more or less powerfrom the transmit antenna than the DC open state or the DC short state.

Reverse link signaling may be accomplished by switching between the DCoperating state (FIG. 13C) and the DC short state (FIG. 13B). Reverselink signaling also may be accomplished by switching between the DCoperating state (FIG. 13C) and the DC open state (FIG. 13A).

FIGS. 14A-14C shows a simplified schematic of a portion of alternativereceive circuitry in various states to illustrate messaging between areceiver and a transmitter.

All of FIGS. 14A-14C show the same circuit elements with the differencebeing state of the various switches. A receive antenna 304 includes acharacteristic inductance L1, which drives node 350. Node 350 isselectively coupled to ground through capacitor C1 and switch S1B. Node350 is also AC coupled to diode D1 and rectifier 318 through capacitorC2. The diode D1 is coupled to a transmit signal 320 which is filteredto remove harmonics and unwanted frequencies with capacitor C3 andresistor R1. Thus the combination of D1, C3, and R1 can generate asignal on the transmit signal 320 that mimics the transmit modulationgenerated by the transmit modulation signal 224 discussed above withreference to the transmitter in FIG. 12.

The rectifier 318 is connected to switch S2B, which is connected inseries with resistor R2 and ground. The rectifier 318 also is connectedto switch S3B. The other side of switch S3B supplies a DC power signal322 to a receive device (not shown) to power the receive device, chargea battery, or a combination thereof.

In FIGS. 13A-13C the DC impedance of the receive antenna 304 is changedby selectively coupling the receive antenna to ground through switchS1B. In contrast, in the embodiments of FIGS. 14A-14C, the impedance ofthe antenna can be modified to generate the reverse link signaling bymodifying the state of switches S1B, S2B, and S3B to change the ACimpedance of the receive antenna 304. In FIGS. 14A-14C the resonantfrequency of the receive antenna 304 may be tuned with capacitor C2.Thus, the AC impedance of the receive antenna 304 may be changed byselectively coupling the receive antenna 304 through capacitor C1 usingswitch S1B, essentially changing the resonance circuit to a differentfrequency that will be outside of a range that will optimally couplewith the transmit antenna. If the resonance frequency of the receiveantenna 304 is near the resonant frequency of the transmit antenna, andthe receive antenna 304 is in the near-field of the transmit antenna, acoupling mode may develop wherein the receiver can draw significantpower from the radiated field 106.

In FIG. 14A, switch S1B is closed, which de-tunes the antenna andcreates an “AC cloaking state,” essentially “cloaking” the receiveantenna 304 from detection by the transmit antenna 204 because thereceive antenna does not resonate at the transmit antenna's frequency.Since the receive antenna will not be in a coupled mode, the state ofswitches S2B and S3B are not particularly important to the presentdiscussion.

In FIG. 14B, switch S1B is open, switch S2B is closed, and switch S3B isopen, creating a “tuned dummy-load state” for the receive antenna 304.Because switch S1B is open, capacitor C1 does not contribute to theresonance circuit and the receive antenna 304 in combination withcapacitor C2 will be in a resonance frequency that may match with theresonant frequency of the transmit antenna. The combination of switchS3B open and switch S2B closed creates a relatively high current dummyload for the rectifier, which will draw more power through the receiveantenna 304, which can be sensed by the transmit antenna. In addition,the transmit signal 320 can be detected since the receive antenna is ina state to receive power from the transmit antenna.

In FIG. 14C, switch S1B is open, switch S2B is open, and switch S3B isclosed, creating a “tuned operating state” for the receive antenna 304.Because switch S1B is open, capacitor C1 does not contribute to theresonance circuit and the receive antenna 304 in combination withcapacitor C2 will be in a resonance frequency that may match with theresonant frequency of the transmit antenna. The combination of switchS2B open and switch S3B closed creates a normal operating state whereinpower can be supplied by the DC out signal 322 and a transmit signal 320can be detected.

Reverse link signaling may be accomplished by switching between thetuned operating state (FIG. 14C) and the AC cloaking state (FIG. 14A).Reverse link signaling also may be accomplished by switching between thetuned dummy-load state (FIG. 14B) and the AC cloaking state (FIG. 14A).Reverse link signaling also may be accomplished by switching between thetuned operating state (FIG. 14C) and the tuned dummy-load state (FIG.14B) because there will be a difference in the amount of power consumedby the receiver, which can be detected by the load sensing circuit inthe transmitter.

Of course, those of ordinary skill in the art will recognize that othercombinations of switches S1B, S2B, and S3B may be used to createcloaking, generate reverse link signaling and supplying power to thereceive device. In addition, the switches S1A and S1B may be added tothe circuits of FIGS. 14A-14C to create other possible combinations forcloaking, reverse link signaling, and supplying power to the receivedevice.

Thus, when in a coupled mode signals may be sent from the transmitter tothe receiver, as discussed above with reference to FIG. 12. In addition,when in a coupled mode signals may be sent from the receiver to thetransmitter, as discussed above with reference to FIGS. 13A-13C and14A-14C.

FIGS. 15A-15C are timing diagrams illustrating a messaging protocol forcommunication between a transmitter and a receiver using the signalingtechniques discussed above. In one exemplary approach, signals from thetransmitter to the receiver are referred to herein as a “forward link”and use a simple AM modulation between normal oscillation and nooscillation. Other modulation techniques are also contemplated. As anon-limiting example, a signal present may be interpreted as a 1 and nosignal present may be interpreted as a 0.

Reverse link signaling is provided by modulation of power drawn by thereceive device, which can be detected by the load sensing circuit in thetransmitter. As a non-limiting example, higher power states may beinterpreted as a 1 and lower power states may be interpreted as a 0. Itshould be noted that the transmitter must be on for the receiver to beable to perform the reverse link signaling. In addition, the receivershould not perform reverse link signaling during forward link signaling.Furthermore, if two receive devices attempt to perform reverse linksignaling at the same time a collision may occur, which will make itdifficult, if not impossible for the transmitter to decode a properreverse link signal.

In the exemplary embodiment described herein, signaling is similar to aUniversal Asynchronous Receive Transmit (UART) serial communicationprotocol with a start bit, a data byte, a parity bit and a stop bit. Ofcourse, any serial communication protocol may be suitable for carryingthe exemplary embodiment of the present invention described herein. Forsimplicity of description, and not as a limitation, the messagingprotocol will be described such that the period for communicating eachbyte transmission is about 10 mS.

FIG. 15A illustrates the simplest, and lowest power form of themessaging protocol. A synchronization pulse 420 will be repeated everyrecurring period 410 (about one second in the exemplary embodiment). Asa non-limiting example, the sync pulse on time may be about 40 mS. Therecurring period 410 with at least a synchronization pulse 420 may berepeated indefinitely while the transmitter is on. Note that“synchronization pulse” is somewhat of a misnomer because thesynchronization pulse 350 may be a steady frequency during the pulseperiod as illustrated by the “white” pulse 420′. The synchronizationpulse 420 may also include signaling at the resonant frequency with theON/OFF keying discussed above and as illustrated by the “hatched” pulse420. FIG. 15A illustrates a minimal power state wherein power at theresonant frequency is supplied during the synchronization pulse 420 andthe transmit antenna is off during a power period 450. All receivedevices are allowed to receive power during the synchronization pulse420.

FIG. 15B illustrates the recurring period 410 with a synchronizationpulse 420, a reverse link period 430 and a power period 450′ wherein thetransmit antenna is on and supplying full power by oscillating at theresonant frequency and not performing any signaling. The upper timingdiagram illustrates the entire recurring period 410 and the lower timingdiagram illustrates an exploded view of the synchronization pulse 420and the reverse link period 430. The power period 450′ may be segmentedinto different periods for multiple receive devices as is explainedbelow. FIG. 15B shows three power segments Pd1, Pd2, and Pdn for threedifferent receive devices.

When forward link signaling occurs, the synchronization pulse 420 mayinclude a warm-up period 422, a forward link period 424, and a listeningperiod 426. The listening period 426 may include a handover period 427and a beginning reverse link period 428. During the synchronizationpulse 420, the transmitter may send out a forward link message duringthe forward link period 400 (indicated by the “hatched” section) andwaits for a reply from a receiver during the listening period 426. InFIG. 15B, no receivers reply, which is indicated by the “white” sectionsduring the listening period 426.

FIG. 15C is similar to FIG. 15B except that a receiver replies duringthe beginning reverse link period 428 and the reverse link period 430,as indicated by the “cross-hatched” sections. In FIG. 15, during thesynchronization pulse 420, the transmitter sends out a forward linkmessage during the forward link period 400 and waits for a reply from areceiver during the listening period 426. Any receivers that are goingto reply begin their reply before the end of the handover period 427,during the beginning reverse link period 428, and possibly during thereverse link period 430.

As a non-limiting example, Table 2 shows some possible messages that maybe sent by the transmitter and the receiver.

TABLE 2 TX Command TX message RX Reply RX message Null NDQ (New DeviceNDR (New DD TT PP rr cc Query) Device Response) DQ (Device Query) DD DS(Device DD TT PP cc Status) ACK (Acknowledge a device XX from previousDS) SA (Slot Assignment) DD NN MM cc RES (Reset all power slotassignments)

Where:

Null=no transmit command;

DD=Device number;

TT=Device Type;

PP=Power requested;

rr=a random number;

cc=a checksum;

NN=start of time slot; and

MM=end of time slot

In explaining table 1, the null command means that no messaging is sentby the transmitter during the forward link period 424. In line 2, a newdevice query (NDQ) is sent by the transmitter. If a receive deviceresponds, it responds with a new device response (NDR) along with adevice number (which should be zero for a new device, until the devicenumber is assigned by the transmitter), a power request, a randomnumber, and a checksum of all the data bits in the receive reply.

In line 3, a new device query (DQ) is sent by the transmitter along witha device number. The receive device that was addressed by the DQ replieswith a device status (DS), along with the device number, the devicetype, the amount of power requested, and a checksum of all the data bitsin the receive reply.

In line 4, the transmitter sends out an acknowledge (ACK) to thereceiver that replied to the previous DQ. No receivers respond to an ACK

In line 5, the transmitter sends out a slot assignment (SA) along with adevice number, a start time within the power period 450′, an end timewithin the power period 450′, and a checksum of all the data bits in thereceive reply. No receivers respond to an SA.

In line 6, the transmitter sends out a reset (RES) indicating that allreceivers should stop using their allocated time slots. No receiversrespond to an RES.

Of course, those of ordinary skill in the art will recognize that thecommands and responses are exemplary and various embodimentscontemplated within the scope of the present invention may usevariations of these commands and responses, and additional commands andresponses may be devised within the scope of the present invention.

To further illustrate how communication occurs, five different scenarioswill be discussed. In the first scenario, initially no receive devicesare within the coupling-mode region of the transmitter and one receivedevice enters the coupling-mode region. When no device are present inthe coupling-mode region the transmitter will remain in the low powerstate as illustrated in FIG. 15A and repeat the synchronization pulse420 every recurring period 410. The synchronization pulse 420 willinclude a NDQ during the forward link period 424 and the transmitterwill listen for a reply during the listening period 426. If no reply isreceived, the transmitter shuts down until time for the synchronizationpulse 420 of the next recurring period 410.

When a new receive device is introduced to the coupling-mode region, thereceive device is initially on and listening for a synchronization pulse420. The new receive device may use the synchronization pulse 420 forpower but should go into a cloaked or non-power reception mode (referredto herein as “getting off the bus”) during the power period 450′. Inaddition, the new receive device listens for transmit commands andignores all transmit commands except an NDQ. When a new receive devicereceive an NDQ, it remains on during the handover period 427, thebeginning reverse link period 428, and possibly the reverse link period430. After the forward link period 424 and before the end of thehandover period 427, the receive device responds with a NDR, a device IDof zero (a new device ID will be assigned by the transmitter), a poweramount request, a random number and a checksum. The new receive devicethen gets off the bus during the power period 450′.

If the transmitter receives the NDR correctly, it responds on the nextsynchronization pulse 420 with a slot assignment (SA) for the newreceive device. The SA includes a device ID for the new receive device,a start time, an end time, and a checksum. The start time and end timefor this SA will be zero indicating that the new receive device shouldnot get on the bus for any time period during the power period 450′. Thenew receive device will receive a subsequent SA with actual start timesand end times assigning a specific power segment Pdn when it can get onthe bus. If the new receive device does not receive a proper checksum,in remains in new device mode and responds again to an NDQ.

In the second scenario, no receive devices are within the coupling-moderegion of the transmitter and more than one receive device enters thecoupling-mode region. In this mode, when two new receive devices areintroduced to the coupling-mode region they are initially on the bus allthe time. The new receive devices may use the synchronization pulse 420for power but should get off the bus during the power period 450′ once asynchronization pulse 420 has been received. In addition, the newreceive devices listen for transmit commands and ignore all transmitcommands except an NDQ. When the new receive device receive an NDQ, theyremain on during the handover period 427, the beginning reverse linkperiod 428, and possibly the reverse link period 430. After the forwardlink period 424 and before the end of the handover period 427, thereceive devices responds with a NDR, a device ID of zero (a new deviceID will be assigned by the transmitter), a power amount request, arandom number and a checksum.

However, since two or more receive devices are responding at the sametime, and likely have different random numbers and checksums, themessage received by the transmitter will be garbled, and the checksum inthe transmitter will not be accurate. As a result, the transmitter willnot send out a SA on the subsequent synchronization pulse 420.

When an immediate SA is not forthcoming after an NDR, each of thereceive devices waits a random number of subsequent NDQs beforeresponding with an NDR. For example, two devices both respond to thefirst NDQ so no subsequent SA happens. Device 1 decides to wait fourNDQs before responding to another NDQ. Device 2 decides to wait two NDQsbefore responding to another NDQ. As a result, on the next NDQ sent outby the transmitter, neither device responds with an NDR. On the next NDQsent out by the transmitter, only device 2 responds with an NDR, thetransmitter successfully receives the NDR and sends out an SA for device2. On the next NDQ, device 2 does not respond because it is no longer anew device and device 1 does not respond because its random waitingperiod has not elapsed. On the next NDQ sent out by the transmitter,only device 1 responds with an NDR, the transmitter successfullyreceives the NDR and sends out an SA for device 1.

In the third scenario, at least one receive device is in thecoupling-mode region and a new receive device enters the coupling-moderegion. In this mode, the new receive devices is introduced to thecoupling-mode region and is initially on the bus all the time. The newreceive devices may use the synchronization pulse 420 for power butshould get off the bus during the power period 450′ once asynchronization pulse 420 has been received. In addition, the newreceive devices listen for transmit commands and ignore all transmitcommands except an NDQ. Periodically, the transmitter will issue an NDQto see if any new devices have entered the coupling-mode region. The newdevice will then reply with an NDR. On the subsequent synchronizationpulse 420, the transmitter will issue an SA for the new device with nopower slots assigned. The transmitter then recalculates power allocationfor all the devices in the coupling-mode region and generates new SAsfor each device so there are no overlapping power segments Pdn. Aftereach device receives its new SA, it begins getting on the bus onlyduring its new Pdn.

In the fourth scenario, normal power delivery operation continues withno receive device entering or leaving the coupling-mode region. Duringthis scenario, the transmitter will periodically ping each device with adevice query (DQ). The queried device responds with a device status(DS). If the DS indicates a different power request, the transmitter mayreallocate power allocation to each of the devices in the coupling-moderegion. The transmitter will also periodically issues an NDQ as wasexplained above for the third scenario.

In the fifth scenario, a device is removed from the coupling-moderegion. This “removed” state may be that the device is physicallyremoved from the coupling-mode region, the device is shut off, or thedevice cloaks itself, perhaps because it does not need any more power.As stated earlier, the transmitter periodically sends out a DQ for allthe devices in the coupling-mode region. If two consecutive DQs to aspecific device do not return a valid DS, the transmitter removes thedevice from its list of allocated devices and reallocates the powerperiod 450′ to the remaining devices. The transmitter will also assignthe missing device a power allocation of zero time in case it is stillreceiving by is unable to transmit. If a device was erroneously removedfrom the power allocation, it may regain power allocation by respondingto and NDQ with a proper NDR.

Table 3 illustrates a non-limiting sequence of commands and replies toillustrate how the communication protocol operates.

TABLE 3 Command Description Reply Description Comments DQ1 Query Device1 DS 1 1 FF cc Device 1 is Cellphone with low type 1, wants battery maxpower DQ2 Query Device 2 DS 2 1 84 cc Device 2 is PDA with almost type3, wants to charged battery reduce power time slot SA 2 84 Slot assignReduce device 2's FF device 2 power slot (reduce first, then increase)SA 1 00 83 Slot assign Increase device 1's device 1 power slot NDQ Newdevice NDR 00 04 FF rr cc New device Mouse with a low query foundbattery, max power SA 3 00 00 Slot assign Immediate reply after device 3NDQ means it is for new device. Device ID is 3. Initial power slot is 0.SA 1 00 40 Slot assign Device 1 reassigned device 1 to ¼ power. SA 2 4180 Slot assign Device 2 reassigned device 2 to ¼ power. SA 3 81 Slotassign Device 3 reassigned FF device 2 to ½ power. NDQ New device Noreply so no new query device found. null DQ1 DQ2 DQ3 NDQ

Note that the first slot assignment for the new device allocates no timeslot. Each existing device is allocated a new non-overlapping time slot,then the new device is finally allocated a time slot for receivingpower.

In an exemplary embodiment, a wireless charging devices may display avisible signal, such as, for example, a light to the user indicatingthat it has successfully entered the charging region and registereditself to the local transmitter. This will give the user positivefeedback that a device is indeed prepared to charge.

In other exemplary embodiments of the present invention, the receiverand transmitter may communicate on a separate communication channel 119(e.g., Bluetooth, zigbee, cellular, etc) as is shown in FIG. 2. With aseparate communication channel, the recurring period need not includeany communication periods and the entire time may be devoted to thepower period 450′. The transmitter may still allocate time slots to eachreceive device (communicated over the separate communication channel)and each receive device only gets on the bus for its allocated powersegment Pdn.

The time-multiplexed power allocations described above may be themost-efficient method for supplying power to multiple receive deviceswithin a transmitter's coupling-mode region. However, other powerallocation scenarios may be employed with other embodiments of thepresent invention.

FIGS. 16A-16D are simplified block diagrams illustrating a beacon powermode for transmitting power between a transmitter and a one or morereceivers. FIG. 16A illustrates a transmitter 520 having a low power“beacon” signal 525 when there are no receive devices in the beaconcoupling-mode region 510. The beacon signal 525 may be, as anon-limiting example, such as in the range of 10 to 20 mW RF. Thissignal may be adequate to provide initial power to a device to becharged when it is placed in the coupling-mode region.

FIG. 16B illustrates a receive device 530 placed within the beaconcoupling-mode region 510 of the transmitter 520 transmitting the beaconsignal 525. If the receive device 530 is on and develops a coupling withthe transmitter it will generate a reverse link coupling 535, which isreally just the receiver accepting power from the beacon signal 525.This additional power, may be sensed by the load sensing circuit 216(FIG. 12) of the transmitter. As a result, the transmitter may go into ahigh power mode.

FIG. 16C illustrates the transmitter 520 generating a high power signal525′ resulting in a high power coupling-mode region 510′. As long as thereceive device 530 is accepting power and, as a result, generating thereverse link coupling 535, the transmitter will remain in the high powerstate. While only one receive device 530 is illustrated, multiplereceive devices 530 may be present in the coupling-mode region 510. Ifthere are multiple receive device 530 they will share the amount ofpower transmitted by the transmitter based on how well each receivedevice 530 is coupled. For example, the coupling efficiency may bedifferent for each receive device 530 depending on where the device isplaced within the coupling-mode region 510 as was explained above withreference to FIGS. 8 and 9.

FIG. 16D illustrates the transmitter 520 generating the beacon signal525 even when a receive device 530 is in the beacon coupling-mode region510. This state may occur when the receive device 530 is shut off, orthe device cloaks itself, perhaps because it does not need any morepower.

As with the time-multiplexing mode, the receiver and transmitter maycommunicate on a separate communication channel (e.g., Bluetooth,zigbee, etc). With a separate communication channel, the transmitter maydetermine when to switch between beacon mode and high power mode, orcreate multiple power levels, based on the number of receive devices inthe coupling-mode region 510 and their respective power requirements.

Exemplary embodiments of the invention include enhancing the couplingbetween a relatively large transmit antenna and a small receive antennain the near field power transfer between two antennas throughintroduction of additional antennas into the system of coupled antennasthat will act as repeaters and will enhance the flow of power from thetransmitting antenna toward the receiving antenna.

In an exemplary embodiment, one or more extra antennas are used thatcouple to the transmit antenna and receive antenna in the system. Theseextra antennas comprise repeater antennas, such as active or passiveantennas. A passive antenna may include simply the antenna loop and acapacitive element for tuning a resonant frequency of the antenna. Anactive element may include, in addition to the antenna loop and one ormore tuning capacitors, an amplifier for increasing the strength of arepeated near field radiation.

The combination of the transmit antenna and the repeater antennas in thepower transfer system may be optimized such that coupling of power tovery small receive antennas is enhanced based on factors such astermination loads, tuning components, resonant frequencies, andplacement of the repeater antennas relative to the transmit antenna.

A single transmit antenna exhibits a finite near field coupling moderegion. Accordingly, a user of a device charging through a receiver inthe transmit antenna's near field coupling mode region may require aconsiderable user access space that would be prohibitive or at leastinconvenient. Furthermore, the coupling mode region may diminish quicklyas a receive antenna moves away from the transmit antenna.

A repeater antenna may refocus and reshape a coupling mode region from atransmit antenna to create a second coupling mode region around therepeater antenna, which may be better suited for coupling energy to areceive antenna.

FIG. 17A illustrates a large transmit antenna 710A with a smallerrepeater antenna 720A disposed coplanar with, and within a perimeter of,the transmit antenna 710A. The transmit antenna 710A and repeaterantenna 720A are both formed on a table 740. A device including areceive antenna 730A is placed within the perimeter of the repeaterantenna 720A. With very large antennas, there may be areas of thecoupling mode region that are relatively weak near the center of thetransmit antenna 710A. Presence of this weak region may be particularlynoticeable when attempting to couple to a very small receive antenna730A. The repeater antenna 720A placed coplanar with the transmitantenna 710A, but with a smaller size, may be able to refocus thecoupling mode region generated by the transmit antenna 710A into asmaller and stronger repeated coupling mode region around the repeaterantenna 720A. As a result, a relatively strong repeated near fieldradiation is available for the receive antenna 730A.

FIG. 17B illustrates a transmit antenna 710B with a larger repeaterantenna 720B with a coaxial placement relative to the transmit antenna710B. A device including a receive antenna 730B is placed within theperimeter of the repeater antenna 720B. The transmit antenna 710B isformed around the lower edge circumference of a lamp shade 742, whilethe repeater antenna 720B is disposed on a table 740. Recall that withcoaxial placements, the near field radiation may diminish relativelyquickly relative to distance away from the plane of an antenna. As aresult, the small receive antenna 730B placed in a coaxial placementrelative to the transmit antenna 710B may be in a weak coupling moderegion. However, the large repeater antenna 720B placed coaxially withthe transmit antenna 710B may be able to reshape the coupled mode regionof the transmit antenna 710B to another coupled mode region in adifferent place around the repeater antenna 720B. As a result, arelatively strong repeated near field radiation is available for thereceive antenna 730B placed coplanar with the repeater antenna 720B.

FIG. 18A illustrates a large transmit antenna 710C with three smallerrepeater antennas 720C disposed coplanar with, and within a perimeterof, the transmit antenna 710C. The transmit antenna 710C and repeaterantennas 720C are formed on a table 740. Various devices includingreceive antennas 730C are placed at various locations within thetransmit antenna 710C and repeater antennas 720C. As with the embodimentillustrated in FIG. 17A, the embodiment of FIG. 18A may be able torefocus the coupling mode region generated by the transmit antenna 710Cinto smaller and stronger repeated coupling mode regions around each ofthe repeater antennas 720C. As a result, a relatively strong repeatednear field radiation is available for the receive antennas 730C. Some ofthe receive antennas are placed outside of any repeater antennas 720C.Recall that the coupled mode region may extend somewhat outside theperimeter of an antenna. Therefore, receive antennas 730C may be able toreceive power from the near field radiation of the transmit antenna 710Cas well as any nearby repeater antennas 720C. As a result, receiveantennas placed outside of any repeater antennas 720C, may be still beable to receive power from the near field radiation of the transmitantenna 710C as well as any nearby repeater antennas 720C.

FIG. 18B illustrates a large transmit antenna 710D with smaller repeaterantennas 720D with offset coaxial placements and offset coplanarplacements relative to the transmit antenna 710D. A device including areceive antenna 730D is placed within the perimeter of one of therepeater antennas 720D. As a non-limiting example, the transmit antenna710D may be disposed on a ceiling 746, while the repeater antennas 720Dmay be disposed on a table 740. As with the embodiment of FIG. 17B, therepeater antennas 720D in an offset coaxial placement may be able toreshape and enhance the near field radiation from the transmitterantenna 710D to repeated near field radiation around the repeaterantennas 720D. As a result, a relatively strong repeated near fieldradiation is available for the receive antenna 730D placed coplanar withthe repeater antennas 720D.

While the various transmit antennas and repeater antennas have beenshown in general on surfaces, these antennas may also be disposed undersurfaces (e.g., under a table, under a floor, behind a wall, or behind aceiling), or within surfaces (e.g., a table top, a wall, a floor, or aceiling).

FIG. 19 shows simulation results indicating coupling strength between atransmit antenna, a repeater antenna and a receive antenna. The transmitantenna, the repeater antenna, and the receive antenna are tuned to havea resonant frequency of about 13.56 MHz.

Curve 810 illustrates a measure for the amount of power transmitted fromthe transmit antenna out of the total power fed to the transmit antennaat various frequencies. Similarly, curve 820 illustrates a measure forthe amount of power received by the receive antenna through the repeaterantenna out of the total power available in the vicinity of itsterminals at various frequencies. Finally, Curve 830 illustrates theamount of power actually coupled between the transmit antenna, throughthe repeater antenna and into the receive antenna at variousfrequencies.

At the peak of curve 830, corresponding to about 13.56 MHz, it can beseen that a large amount of the power sent from the transmitter isavailable at the receiver, indicating a high degree of coupling betweenthe combination of the transmit antenna, the repeater antenna and thereceive antenna.

FIG. 20A show simulation results indicating coupling strength between atransmit antenna and receive antenna disposed in a coaxial placementrelative to the transmit antenna with no repeater antennas. The transmitantenna and the receive antenna are tuned to have a resonant frequencyof about 10 MHz. The transmit antenna in this simulation is about 1.3meters on a side and the receive antenna is a multi-loop antenna atabout 30 mm on a side. The receive antenna is placed at about 2 metersaway from the plane of the transmit antenna. Curve 810A illustrates ameasure for the amount of power transmitted from the transmit antennaout of the total power fed to its terminals at various frequencies.Similarly, curve 840 illustrates a measure of the amount of powerreceived by the receive antenna out of the total power available in thevicinity of its terminals at various frequencies. Finally, Curve 830Aillustrates the amount of power actually coupled between the transmitantenna and the receive antenna at various frequencies.

FIG. 20B show simulation results indicating coupling strength betweenthe transmit and receive antennas of FIG. 20A when a repeater antenna isincluded in the system. The transmit antenna and receive antenna are thesame size and placement as in FIG. 20A. The repeater antenna is about 28cm on a side and placed coplanar with the receive antenna (i.e., about0.1 meters away from the plane of the transmit antenna). In FIG. 20B,Curve 810B illustrates a measure of the amount of power transmitted fromthe transmit antenna out of the total power fed to its terminals atvarious frequencies. Curve 820B illustrates the amount of power receivedby the receive antenna through the repeater antenna out of the totalpower available in the vicinity of its terminals at various frequencies.Finally, Curve 830B illustrates the amount of power actually coupledbetween the transmit antenna, through the repeater antenna and into thereceive antenna at various frequencies.

When comparing the coupled power (830A and 830B) from FIGS. 20A and 20Bit can be seen that without a repeater antenna the coupled power 830Apeaks at about −36 dB. Whereas, with a repeater antenna the coupledpower 830B peaks at about −5 dB. Thus, near the resonant frequency,there is a significant increase in the amount of power available to thereceive antenna due to the inclusion of a repeater antenna.

As stated earlier, the receive antennas and transmit antennas aredesigned to operate in the near field of each other where they aretightly coupled to each other and their local environment. This meanscoupling and scattering of RF energy to and from other structures (whichcan include other devices that need charging) can influence the drivingpoint impedance of both the transmit and receive antennas. A poorimpedance match between the transmitter electronics and/or receiveelectronics to their respective antenna ports can cause degradation tothe level of power transfer between the two. Further, since the localenvironment can change with time (devices and structures nearby beingremoved or added), the degradation can also be time dependant. Theexemplary embodiments of the invention have the capability to adaptivelyimprove the impedance match in response to this changing environment,thereby improving the power transfer between devices and the overalltime to charge a device.

Exemplary embodiments of the invention include an RF circuit topologycapable of adaptively optimizing the impedance match between thetransmit electronics and transmit antenna or the receiver electronicsand the receiver antenna. These adaptations are performed in a mannerthat self-adjusts to the local environment to optimize the powercoupling between transmit antennas and receiver antennas.

Exemplary embodiments of the invention use feedback mechanisms andvariable capacitors connected to the antenna to achieve this adaptivetuning function. As non-limiting examples, the adaptive tuning may beused to optimize a coupling of the near field radiation between atransmit antenna and a receive antenna, a transmit antenna and arepeater antenna, a repeater antenna and a receive antenna, orcombinations thereof. In addition, the adaptive tuning function may beused to tune transmit antennas, receive antennas, or combinationsthereof when additional receive antennas are placed in the coupling moderegion of the transmit antenna.

FIG. 21A-21C are simplified block diagrams of adaptive tuning circuitsfor an antenna using a T-network, an L-network, and a Pi-network,respectively. FIGS. 21A-C include an antenna 904 with characteristicimpedance L1 to receive or transmit a radiated field 106. The antenna isconnected to a coupler 910 for splitting the signal between a voltagestanding wave ratio (VSWR) detector 920 and a variable capacitornetwork, which may be designated generically as 950. Specifically, inFIG. 21A the variable capacitor network 950T is configured in aT-network, in FIG. 21B the variable capacitor network 950L is configuredin an L-network, and in FIG. 21C the variable capacitor network 950P isconfigured in a Pi-network. The VSWR detector 920 is connected to acontroller 930, which controls each of the variable capacitors usingcontrol signals. An RF signal 955 couples to additional circuitry (notshown) depending on whether the antenna 904 is part of a receiver,transmitter, or repeater.

In operation, the RF signal from the coupler 910 feeds the VSWR detector920. As a non-limiting example, the VSWR detector 920 may be a diodepower detector circuit, which generates a voltage that is proportionalto the magnitude of the reflected signal. A large reflected signal atthe antenna may indicate that the antenna 904 is operatinginefficiently. This proportional voltage is read by the controller 930.The controller includes software to analyze the characteristics of theL-C network comprising the characteristic impedance L1 of the antenna904 coupled with the variable capacitor network 950. Based on theproportional voltage, the controller 930 uses the control signals toadjust capacitance values of the variable capacitance networks.

In the case of FIG. 21A, the control signals may adjust capacitance forvariable capacitors VC1A, VC2A, and VC3A configured in a T-network. Inthe case of FIG. 21B, the control signals may adjust capacitance forvariable capacitors VC1B and VC2B configured in an L-network. In thecase of FIG. 21C, the control signals may adjust capacitance forvariable capacitors VC1C, VC2C, and VC3C configured in a Pi-network.

This adjustment of the variable capacitors closes the feedback loop. Theproportional voltage from the VSWR detector 920 adjusts based on the newreflection characteristics at the antenna 904, the controller 930samples the new proportional voltage and modifies the control signals toadjust the variable capacitor network 905 again. The feedback loopcontinues to monitor and adjust to minimize reflected power at theantenna 904, which maximized the power delivered out of the antenna 904.

FIG. 22 is a simplified block diagram of an adaptive tuning circuit fora transmit antenna 904 based on power consumption at the transmitantenna 904. An oscillator 922 drives a power amplifier 924, whichreceives power from a PA sensor 940 via a power input Vds. The PA sensor940 feeds a controller 930, which generates control signals to adjustthe variable capacitor (VC1C, VC2C, and VC3C) in the variable capacitornetwork 950P.

Instead of monitoring the change in reflected energy at the transmitantenna, the embodiment of FIG. 22 detects changes in the power consumedfrom the near field radiation 106. As the power consumed changes,reflected energy at the antenna may change, which may be minimized byadjusting the variable capacitor network 950P. The feedback mechanismoperates similarly as for the embodiments of FIGS. 21A-21C, except thatconsumed power is monitored to determine antenna inefficiencies ratherthan a VSWR. Of course, while not illustrated, those of ordinary skillin the art will recognize any of the variable capacitor networksillustrated in FIGS. 21A-21C may be used in the FIG. 22 embodiment.Furthermore, other suitable variable capacitor networks may be used withany of the embodiments of FIGS. 21A-21C and 22.

FIGS. 23A and 23B are simplified circuit diagrams illustrating exemplaryembodiments of variable capacitor networks. In FIG. 23A, the variablecapacitors are implemented as switched capacitors configured in anL-network 950L. Thus, the L(RX/TX) is the characteristic impedance ofthe antenna, C11, C12, . . . C1n form the variable capacitor VC1B, andC21, C22, . . . C2m form the variable capacitor VC2B. The Z(load) modelsany load on the network. A 1:n decoder controls the switches to the VC1Bcapacitors and a 1:m decoder controls the switches to the VC2Bcapacitors. Thus, a variable amount of capacitance may be selected. Thecapacitors may be configured to all have the same capacitance, they maybe configured to have binary weights, or they may be configured withother suitable weights.

In FIG. 23B, the variable capacitors are implemented as varactors (i.e.,reverse biased variable capacitance diodes) configured in a T-network950T. Thus, the control signals 935 are analog signals used to adjustthe capacitance of variable capacitors VC1A, VC2A, and VC3A by adjustingthe bias to diodes V11, V12, V21, V22, V31, and V32. As the bias isadjusted, the apparent capacitance of the combined reverse diodes (i.e.,V11-V12, V21-V22, and V31-V32) adjusts.

Other types of variable capacitors may be used to implement the variablecapacitor networks. As a non-limiting example, variable capacitorMicro-Electro-Mechanical System (MEMS) variable capacitors have beenrecently proposed, which may be used as the variable capacitors for anyof the variable capacitor networks. Furthermore, more than one type ofvariable capacitor may be used in any given variable capacitor network.

FIGS. 24A and 24B illustrates simulation results for near field coupledtransmit and receive antennas before and after adaptive tuning,respectively. In FIG. 24A, curve 860A illustrates the amount of powernot transmitted from the transmit antenna at various frequencies (i.e.,return loss). Similarly, curve 870A illustrates the amount of power notreceived by the receive antenna at various frequencies (i.e., returnloss). Finally, curve 880A illustrates the amount of power actuallycoupled between the transmit antenna and the receive antenna at variousfrequencies. As can be seen in FIG. 24A, the transmit antenna is poorlytuned as illustrated by the relatively minor dip in the curve at theresonant frequency of about 13.56 MHz.

In FIG. 24B, curve 860B illustrates the amount of power not transmittedfrom the transmit antenna at various frequencies. Similarly, curve 870Billustrates the amount of power not received by the receive antenna atvarious frequencies. Finally, curve 880B illustrates the amount of poweractually coupled between the transmit antenna and the receive antenna atvarious frequencies. As can be seen, the transmit antenna is tuned muchbetter as indicated by the significant dip at the resonant frequency ofabout 13.56 MHz. The coupled energy indicated by curve 880B is notsignificantly higher in FIG. 24B relative to the coupled energy 880A inFIG. 24A. However, this may be due to other factors, such as, forexample, the presence of another receive antenna, the placement of thereceive antenna relative to the transmit antenna, or an improperly tunedreceive antenna.

Thus, in systems with adaptive tuning in both the receive antenna andthe transmit antenna, both antennas will be continuously tuned to adaptto changes in the environment, and even tuning changes at the otherantenna. For example, in FIG. 24A curve 860A indicates that the receiveantenna is not as well tuned as in FIG. 24B illustrated by curve 860B.This, change in tuning may be a result of the change in tuning in thetransmit antenna. As a result, a receive antenna with adaptive tuningwould attempt to achieve better tuning, resulting in more couplingbetween the transmit and receive antenna.

As stated earlier, an adaptive tuner network similar to those used withthe receive and transit antennas can be used to further enhance thepower coupling between transmit, receive, and repeater antennas. In thecase of the repeater, RF-DC conversion circuitry (as illustrated byelements 308 and 310 in FIG. 11) may be integrated in the repeaterantenna structure and can be used to convert RF power to DC power. ThisDC power may be used as the power source to control the repeateradaptive tuning network as well as other active elements in an activerepeater. In this way, there may be no tethered power sources to anactive repeater antenna. In an exemplary embodiment, a series of 3-4LEDs may be integrated on the repeater and if practical, on the receiverantennas to indicate when each is in the range of the transmitterantenna.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the exemplary embodiments of the invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a Digital SignalProcessor (DSP), an Application Specific Integrated Circuit (ASIC), aField Programmable Gate Array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in Random Access Memory (RAM), flashmemory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM),Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, aremovable disk, a CD-ROM, or any other form of storage medium known inthe art. An exemplary storage medium is coupled to the processor suchthat the processor can read information from, and write information to,the storage medium. In the alternative, the storage medium may beintegral to the processor. The processor and the storage medium mayreside in an ASIC. The ASIC may reside in a user terminal. In thealternative, the processor and the storage medium may reside as discretecomponents in a user terminal.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by acomputer. By way of example, and not limitation, such computer-readablemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium that can be used to carry or store desired program code inthe form of instructions or data structures and that can be accessed bya computer. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosed exemplary embodiments isprovided to enable any person skilled in the art to make or use thepresent invention. Various modifications to these exemplary embodimentswill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other embodiments withoutdeparting from the spirit or scope of the invention. Thus, the presentinvention is not intended to be limited to the embodiments shown hereinbut is to be accorded the widest scope consistent with the principlesand novel features disclosed herein.

What is claimed is:
 1. A wireless power repeater, comprising: a powertransfer component configured to receive power wirelessly from atransmitter external to the repeater at a level sufficient for chargingan electronic device, the power transfer component further configured totransmit power wirelessly to a receiver external to the repeater at thelevel sufficient for charging the electronic device; a voltage standingwave ratio detector circuit operably coupled to the power transfercomponent and configured to generate a mismatch signal indicating adegree of mismatch at the power transfer component based on the receivedpower; a controller configured to analyze the mismatch signal and togenerate control signals responsive thereto; and a variable capacitoroperably coupled between the power transfer component and thecontroller, the variable capacitor having a capacitance and configuredto modify the capacitance based on the control signals.
 2. The wirelesspower repeater of claim 1, wherein the variable capacitor comprises anetwork selected from the group consisting of a T-network, an L-network,and a Pi-network.
 3. The wireless power repeater of claim 1, wherein thevariable capacitor comprises a plurality of variable capacitors, eachvariable capacitor controlled by one of the control signals, wherein acapacitance of each capacitor is adjusted based on its correspondingcontrol signal.
 4. The wireless power repeater of claim 3, wherein eachof the plurality of variable capacitors comprises a plurality ofswitched capacitors operably coupled in parallel, each switchedcapacitor comprising a switch and a capacitor operably coupled inseries, wherein each switched capacitor is selectively included in thevariable capacitor based on the control signal operatively coupledthereto.
 5. The wireless power repeater of claim 3, wherein each of theplurality of variable capacitors comprises at least one varactoroperatively coupled to a corresponding control signal acting as a biascontrol for the at least one varactor, the at least one varactorselected from the group consisting of a reverse bias varactor diode anda MEMS variable capacitor.
 6. The wireless power repeater of claim 1,wherein the power transfer component and the transmit power transfercomponent each comprise a coil.
 7. The wireless power repeater of claim1, wherein the power transfer component and the transmit power transfercomponent each comprise an antenna.
 8. The wireless power repeater ofclaim 1, wherein the variable capacitor is configured to modify thecapacitance of the variable capacitor to modify a resonance of the powertransfer component and the variable capacitor.
 9. A method of repeatingtransmission of wireless power, comprising: resonating a power transfercomponent of a receiver antenna of a repeater substantially near aresonant frequency of a transmitter external to the repeater; receivingpower wirelessly from the transmitter at a level sufficient for chargingan electronic device; resonating the power transfer componentsubstantially near a resonant frequency of a receiver external to therepeater; transmitting power wirelessly to the receiver at the levelsufficient for charging the electronic device; and adaptively tuning thepower transfer component, the adaptively tuning comprising: detecting amismatch based on a voltage standing wave ratio at the power transfercomponent; generating a mismatch signal based on the detected mismatch;and modifying a capacitance of a variable capacitor based on themismatch signal.
 10. The method of claim 9, wherein the power transfercomponent comprises a tunable power transfer component.
 11. The methodof claim 10, wherein the tunable power transfer component and the powertransfer component are coils.
 12. The method of claim 10, wherein thetunable power transfer component and the power transfer component areantennas.
 13. The method of claim 9, wherein modifying the capacitanceof the variable capacitor comprises selectively enabling a plurality ofswitched capacitors responsive to the mismatch signal, wherein theplurality of switched capacitors are operably coupled to form thevariable capacitor.
 14. The method of claim 9, wherein modifying thecapacitance of the variable capacitor comprises biasing a plurality ofvaractors based on the mismatch signal, wherein the plurality ofvaractors are operably coupled to form the variable capacitor network.15. The method of claim 9, wherein the power transfer componentresonates over a coupling-mode region.
 16. The method of claim 9,wherein modifying the capacitance comprises modifying a resonance of thetunable power transfer component.
 17. A wireless power transfer system,comprising: means for receiving power wirelessly from a transmitterexternal to the wireless power transfer system, the transmitterresonating substantially near a first resonant frequency; means fortransmitting power wirelessly to a receiver external to the wirelesspower transfer system, the receiver resonating substantially near asecond resonant frequency; and means for adaptively tuning the receivingpower wirelessly means, the adaptively tuning means comprising: meansfor detecting a mismatch based on a voltage standing wave ratio at thereceiving power wirelessly means; means for generating a mismatch signalbased on the detected mismatch; and means for modifying a capacitance ofa variable capacitor based on the mismatch signal.
 18. The system ofclaim 17, wherein the receiving power wirelessly means comprises a powertransfer component, wherein the transmitting power wirelessly meanscomprises a transmit power transfer component, wherein the adaptivelytuning means comprises a controller, wherein the mismatch detectingmeans comprises a voltage standing wave ratio detector circuit, whereinthe generating mismatch signal means comprises the voltage standing waveratio detector circuit, and wherein the modifying capacitance meanscomprises a variable capacitor.
 19. The system of claim 18, wherein thepower transfer component comprises a tunable power transfer component.20. The system of claim 19, wherein the tunable power transfercomponent, power transfer component, and the transmit power transfercomponent each comprise a coil.
 21. The system of claim 19, wherein thetunable power transfer component, power transfer component, and thetransmit power transfer component each comprise an antenna.
 22. Thesystem of claim 19, wherein the means for modifying the capacitancecomprises means for modifying a resonance of the tunable power transfercomponent.
 23. The system of claim 18, wherein the modifying capacitancemeans comprises selectively enabling a plurality of switched capacitorsbased on the mismatch signal, wherein the plurality of switchedcapacitors are operably coupled to form the variable capacitor.
 24. Thesystem of claim 18, wherein the modifying capacitance means comprisesbiasing a plurality of varactors based on the mismatch signal, whereinthe plurality of varactors are operably coupled to form the variablecapacitor.